1. Field of the Invention
The present invention relates to an improved process for the preparation of acylferrocenes from ferrocenes using acid anhydrides as acylating agents and montmorillonite clays as catalysts.
2. Summary of the Related Art
Acylferrocenes are important intermediates for the production of varied functional materials such as functional polymers, surfactants, charge transfer complexes, ion sensors, masking agents, coupling agents, chiral catalysts, combustion catalysts for propellants, and the like. Acylferrocenes are also employed as a medicament for the treatment of sideropenia symptoms and sideropenic anemia (anemia caused by repeated bloodletting).
Friedel-Crafts acylation of ferrocene with acyl halides leads to the formation of acylferrocenes. Ferrocene is 10.sup.6 times more reactive than benzene in acylation. They are generally prepared by reacting ferrocene with acetic anhydride/acyl chlorides in the presence of a Lewis acid such as aluminium chloride or polyphosphoric acid. It has been found that both acylferrocenes and diacylferrocenes can be prepared in satisfactory yields by varying the ratios of ferrocene and/or acid chloride, and by altering the mode of addition as well. Monoacylferrocenes are prepared by the drop-wise addition of the acid chloride-aluminium chloride complex to the ferrocene solution, using equimolar amounts of acid chloride, catalyst and ferrocene. The diacylferrocenes are prepared by adding ferrocene solution to the complex using a molar ratio of &gt;2:1 of both the acid chloride and aluminium chloride to ferrocene.
Rosenblum and Woodward, J. Am. Chem. Soc., 80 5443, (1958) discloses the preparation of acylferrocenes by the reaction of ferrocene with acetyl chloride in the presence of stoichiometric quantities of aluminum chloride. "Vogels Textbook of Practical Organic Chemistry", 5.sup.th Edition, 1014 (1989) teaches the preparation of acylferrocene by the reaction of ferrocene with acetic anhydride in the presence of 85% phosphoric acid.
Furthermore, British Patent Nos. BP 869504 and BP 819108 disclose the reaction of ferrocene with an appropriate acid chloride or anhydride under Friedel-Crafts conditions in the presence of Lewis acids such as aluminium chloride, boron trifluoride, hydrogen fluoride or polyphosphoric acid in solvents such as carbon disulfide, ethers, nitromethane or ethylene chloride. The drawbacks in these processes are that when aluminium chloride is used, diacyl compounds are usually obtained, whereas in the presence of hydrogen fluoride or polyphosphoric acid, monoacyl compounds are produced. Other drawbacks are the use of stoichiometric quantities of hazardous aluminium chloride or phosphoric acid and tedious work-up procedures.
It is thus necessary to find a novel process for making acylferrocenes that eliminates the use of the corrosive Lewis acids such as aluminum chloride. It is further desirable to find such a process that is more economical and more selective towards a particular product.